In the current general trend towards miniaturization, integrated micro-optical elements have played a central role in data storage, optical displays and imaging systems. Fine alignment and focus adjustment in these systems is usually performed by means of mechanical parts that are often expensive, fragile and slow.
To overcome limitations introduced by mechanical adjustment, different electrical based inventions have been proposed. A first strategy relies on reshaping the surface of a liquid using an electrical signal without altering the thermo-optical properties of the material. For instance, the electro wetting lens described in U.S. Patent Application Publication No. 2013/0194323 A1 demonstrates that the shape of a liquid drop can be modified by applying a voltage. The local voltage applied to the surface changes the contact angle between the drop and the surface, thus modifying the shape of the drop, which results in a change of the focal point associated to the liquid based micro lens. Nevertheless, such an approach suffers from drawbacks, which include (but are not restricted to) difficulties of integration, slow time response (in some applications, acceleration of the system could alter the shape of the drop and introduce imperfections in the lens) and inability to simultaneously image multiple planes with a single lens.
A second strategy uses an electrical excitation to deliver energy which alters the lensing material, placed between two electrodes, and tunes its properties. Following this strategy, an electrically excited thermo-optical lens as disclosed in U.S. Patent Publication No. 2005/0117195 has been invented. In this patent, a thermo-optical polymer is enclosed between two optically transparent surfaces, one of them curved, and a temperature controller is coupled to the thermo-optical material. This invention allows for the change of focal property of the such formed lens by changing the temperature. Although the entire focal plane of a single lens can be changed this invention does not allow for finer control such as local adjustment of the focal point, resulting in different focal planes induced by a single element. Furthermore, when considering this invention in a matrix configuration, there is a need to electrically address each lens, which could result in a complex engineering of the lens system, prohibiting its use in some applications.
Generally, electrical excitation can be limiting for some applications. To overcome the obstacles posed by mechanical and electrical induced lensing, inventions of optically controlled lenses appear to be relevant. Indeed, optical control eliminates the need to electrically wire each lensing element, and allows for remote control over the lens properties. Furthermore, eliminating the electric wiring or mechanical elements can help improve the transparency of the optical element. Along this line, a first strategy relies on reshaping the surface of a liquid using an optical signal without altering the thermo-optical properties of the material, where light energy is used to modify the surface energy of liquid droplets, which enables to control their shape. For example, liquid drops can be placed on a photo responsive layer, which is placed between a support layer and the droplet. As described in patent EP1304591A1, the droplet can be selectively irradiated with a light source which modifies the surface energy and thus the contact angle between the droplet and the layer, resulting in a variation of the focal length and the lateral position of the focal spot of the micro-lens. This lens still presents most of the drawbacks associated to electro wetting lenses including the ones described previously (difficulty of integration, slow time response and inability to simultaneously image multiple planes with a single lens).